This invention relates to the manufacture of semiconductor devices and, in particular, to semiconductor devices having a silicide/silicon bond.
As known in the art, refractory metal conductive layers are used instead of aluminum or gold in order to obtain a conductor which is more stable during subsequent high temperature process steps. In a particular application, as the gate metal in an MOS transistor, the silicide layer is deposited on a polycrystalline silicon layer and the combination sintered at approximately 900.degree. C.
Because the layers are crystallographically different, and because of the sintering, the layers are under stress. The stress may exceed the adhesion strength of the silicide polycrystalline silicon interface, i.e. the silicide does not adhere well to the polycrystalline silicon. The adhesion problem is not simply one of mechanical stress. Part of the problem is believed to be the formation of what is known as native oxide on the polycrystalline silicon. This oxide is neither intended nor desired, but simply results from exposure of the wafer to an oxygen bearing ambient. The formation of the oxide occurs even at room temperature in air. It is often not thick enough to form a continuous layer.
Known attempts at improving adhesion include careful cleaning of the wafer, forming the silicide layer as soon as possible after forming the polycrystalline silicon layer, adjusting the silicon: metal ratio in the silicide, and reducing the doping level in the polycrystalline silicon, which is normally almost a saturated solid solution and using a silicide with maximum solubility of oxygen and thermodynamic driving force to reduce SiO.sub.2. Even with these measures, adhesion remains a problem, reducing yield. The yield problem shows up as defective die found immediately after manufacture or as defective die found when the device is subjected to temperature cycles. The maximum allowable thickness of the silicide layer depends, although not exclusively, on the amount of stress. Thus, the maximum thickness of an adherent silicide layer becomes a figure of merit, with a thicker layer indicating a better bond or less stress, or both, than a thinner layer.
While improving yield, using any or all of the above techniques to improve adhesion compromises other process parameters. For example, increasing the ratio of silicon to metal improves adhesion but reduces the conductivity of the layer. Decreasing the doping of the polycrystalline silicon improves adhesion but increases electrical resistance of the polycrystalline silicon-silicide interconnects and also increases the contact resistance.